This chapter seeks to familiarize the reader with febrile illnesses. Infections are the dominant cause of childhood focus of infectious (FUOs), whereas neoplasms and connective-tissue disorders are more common in elderly persons. A thorough history, physical examination, and standard laboratory testing remain the basis of the initial evaluation. Newer diagnostic modalities, including evolving rapid molecular diagnostic opportunities and magnetic resonance imaging, have important roles in the assessment of these patients. Acute onset febrile patient who has no infective focus especially in vulnerable age group, consider lumbar puncture (CSF examination) before initiating antimicrobial therapy to avoid ‘false negative’ culture possibility. In newborns (NB) and early childhood, meningitis and septicemia is mostly an index of clinical suspicion and may not present with signs of meningitis such as headache, stiff neck or cerebral dysfunction. Exanthematous febrile illnesses are often diagnosed clinically by recognizing the rash onset, distribution type of skin lesions (macular, papular, vesicular, pustules, petechial, ecchymosis, etc.), and subsequent evolutions of the rash. Major febrile exanthemas and the infection control committee ID surveillance report to DMO are tabulated in this chapter which is self-explanatory.